1. Field of the Invention
This invention relates generally to the processing of periodic information signals, such as video signals, and more particularly is directed to improvements in the write clock pulse signal generator used in a time base corrector for correcting errors introduced during recording and/or reproducing of such periodic information signals.
2. Description of the Prior Art
Video signals are frequently recorded on a magnetic tape and subsequently reproduced for later broadcasting or viewing purposes. During the reproduction of recorded video signals, time base or frequency errors are usually introduced by reason of expansion or contraction of the record medium during or after recording, variation in the speed of the tape relative to the magnetic head or heads during recording or reproduction, variation between the tape recording speed and the tape reproducing speed, and the like. The presence of such time base errors in the reproduced video signals causes a frequency shift of the latter which can result in many observable undesirable effects, particularly when the reproduced video signals are to be transmitted or broadcast and especially if the reproduced video signals are to be mixed with live broadcast material that does not have such time base errors. The observable undesirable effects resulting from relatively small time base errors are a smeared or jittery picture with erroneous intensity variations and, in the case of color video signals, improper color display. When the time base errors are large, the reproduced picture will fail to lock horizontally or vertically.
In an existing time base corrector for substantially removing time base errors from video signals, for example, as disclosed in U.S. Pat. No. 3,860,952, issued Jan. 14, 1975, the incoming video signals are converted from analog to digital form and temporarily stored in a memory. Time base errors are removed from the video signals by writing the digitized signals in the memory at a clocking rate which varies in a manner generally proportional to the time base errors, and by fetching or reading out these stored signals at a standard clocking rate. After such reading out of the digitized video signals, the latter is reconverted to analog form and applied to an output terminal. In the foregoing time base corrector, the rate at which the digitized signals are written in the memory is determined by a write clock pulse signal from a variable frequency or voltage controlled oscillator (VCO) which derives its control voltage from a comparator in which the output of the VCO, after being suitably divided, is compared with the horizontal synchronizing signal separated from the incoming video signals so as to control the frequency of the write clock pulse signal. However, whenever the horizontal synchronizing signals are intermittent or absent because of the occurrence of drop-out, or whenever there is a skew error in the reproduced video signal, the period between the horizontal synchronizing signals becomes much longer than normal. On the other hand, when guard band noise is mixed as a quasi-horizontal synchronizing signal with the normal horizontal synchronizing signal, the period between horizontal synchronizing signals becomes shorter than normal. In such conditions, the frequency of the write clock pulse signal is so varied by the errors in periods between horizontal synchronizing signals that it sometimes becomes impossible to write the digitized signals into memory.